1. Field of the Invention
The invention relates to a coder for incorporating extra information in the form of an auxiliary signal in a digital audio signal having a predetermined format, a decoder for extracting this extra information from the digital signal, a device for recording the digital signal on a record carrier, and a record carrier obtained by means of such a device.
2. Description of the Related Art
In digital sound transmission and recording systems, such as CD players, future television systems, such as D2MAC, and so on, the format, i.e. the sampling rate and the number of bits per sample, by which the digital sound signal is recorded or transmitted, is generally predetermined, for example, by international agreements. Sometimes, however, there is a need for recording or transmitting more information than possible on the basis of the available number of channels. For example, on the basis of international agreements, no more than two high-quality digital audio channels, for example, each channel for 14-bit digital signals, can be available in specific future television systems. These channels are used for transmitting stereophonic audio information for respective left and right-hand channels. However, there is a wish to transmit information for rear channels too, for example, left-hand and a right-hand rear channel for so-called surround sound. Also, in other cases it may be very useful if extra information can be added to existing channels for digital signals having a predetermined format, without the need for extending the number of channels for this purpose. In this context one may think of adding music signals containing music information without vocals, which is commonly referred to as Karaoke, so that the user himself can provide the vocals; or adding music signals in which a specific instrument is omitted, so that the user can play this instrument along with the rest of the recording. One may also think of adding extra information by way of data signals, such as, for example, for Ceefax information.
It will be evident that in all these cases the system is desired to be compatible with existing systems, that is to say, it should be possible to reproduce the original signal information in an undisturbed manner with equipment not comprising a specific decoder for extracting the extra information from the signal. If, for example, there is a television signal containing surround-sound information, in a television set not equipped for producing surround sound, it should be possible to reproduce the information for the left and right-hand channels without this reproduction being disturbed in any audible way by the "masked" information for extracting the signal for the rear channels.